1. Field of the Invention
The present invention relates to a noise suppression apparatus used in radio wave communication and the like, and more particularly to a noise suppression apparatus used for a receiver.
2. Description of the Prior Art
Recently, the noise suppression using digital signal procession technology has been variously developed.
FIG. 20 is a circuit block diagram showing a conventional noise suppression apparatus. In FIG. 20, a reference numeral 20 represents a filter control section, and a reference numeral 52 represents a band pass filter group (abbreviated as BPF group). And a reference numeral 53 represents an adder. The filter control section 51 controls filter factors of the band pass filter group 52 in accordance with the noise level of sound input signal. The band pass filter group 52 includes a group of band pass filters, which separates the sound input signal into a plurality of bands. The characteristics of each band pass filter in this band pass filter group 52 is determined in accordance with the control signal of the filter control section 51. The adder 53 adds output signals fed from these band pass filters of the band pass filter group 52 and produces a noiseless sound signal.
An operation of this conventional noise suppression apparatus is explained below.
An input signal, for example a radio broadcasting sound signal containing-noise, is inputted, into the filter control section 51. The filter control section 51 judges what kind of noise components this input signal contains, and supplies the band pass filter group 52 the filter factors which cut these noise components. The band pass filter group 52 separates the input signal into a plurality of adequate band levels. Each band pass filter in the band pass filter group 52 responds to the filter factor supplied from the filter control section 51 to pass only the sound component of the input signal, and feeds it to the adder 53. The adder 53 adds the output signals of these band pass filters, so as to produce a noiseless sound signal.
In accordance with such a conventional noise suppression apparatus no significant improvement is expected in the articulation, though some increase of the S/N ratio is found. Furthermore, it is recognized that a particular noise sound remains even after the noise suppression is finished by this apparatus.
Moreover, the input signal may contain multipath noises. In order to suppress these multipath noises, there is known a method using a plurality of antennas. In this method, an antenna direction to receive radio waves is different from each other, so that the receiver can select the optimum antenna which is receiving the least multipath noise.
However, in the case where the multipath noise has once received by the receiver, this multipath noise was not removed by the signal processing. Because, the waveform of the radio wave is already distorted when received in the receiver.